Sectorization is a technique that has been used in mobile communications systems in order to minimize multiple access interference (MAI) and hence increase system capacity on each base station [Non-patent Documents 1, 2]. In this technique, a cell of a base station (BS) is divided into a plurality of sectors, and spatially separated mobile units in different sectors can use different channels or share the same channel. Here, the term channel refers to time, frequency, spreading code, interleaving pattern or any other definition as known in communications systems. Furthermore, each sector is serviced by one or more transmit and receive antennas. When adjacent sectors share the same channel, there arises a significant increase in interference among mobile units in the adjacent sectors. This kind of interference is detrimental to wireless communication, and as result, throughput of data transmission between the base station and mobile units will be reduced.
In one of the related art [Patent Document 1], the base station determines an approximate location of a communication unit by comparing uplink signal received from each sector. Comparison is performed on the signal strength of the received uplink signals, although the comparison can be performed using other signal quality metrics such as BER (Bit Error Rate), Word Error Rate, CIR (Committed Information Rate), noise ratio. GPS (Global Positioning System) can also be used. This information can then be used to direct a narrower beam to a target mobile unit, and as result reducing interference. However, in uplink transmission, effect of multiple paths of transmission due to natural terrain will cause the uplink signal to be detected in signals received from multiple sectors. Furthermore, a spatially separated mobile unit may transmit a signal such that part of transmitted signal arrives at a base station in a direction that is very close to a target mobile unit.
FIG. 1 illustrates such a scenario, wherein signals from transmitting unit-1, (TU1) are transmitted to base station (BS) through paths P1 and P2, wherein path (P2) involves a reflection at point B. Similarly, signal from transmitting unit-2, (TU2) is transmitted to the base station (BS) through paths P3.
As can be seen in FIG. 1, at the base station (BS), the direction of arrival of signal along P2 is almost the same as the signal along P3. In this situation, a very narrow beam may have to be created or the signal from this direction will be ignored completely. In practice, very narrow beams can only be created by using a large number of receiving antennas. Creating of such narrow beams ignores part of the transmitted signal that arrives in overlapping directions with interfering signals, hence reducing the power of received signal and as result affecting the throughput of communication.
In another related art, signals from each sector are processed separately [Patent Document 2]. The processing involves demodulation and interference cancellation. Thereafter, the processed signals from the sectors are combined using Maximal Ratio Combining (MRC) [Non-patent Document 5]. Alternative methods of combining or selection can also be deduced from reference [Non-patent Document 6].
FIG. 2 illustrates such kind of a system. By combining signals received from different sectors to generate target signal, overall signal to noise ratio is increased, hence increasing the throughput of a communications system. In addition, this technique of signal processing offers a reduced computational load in comparison to the method of creating beams that are much narrower than the size of a sector.
In multiple receive and transmit antenna system, interference cancellation, which is otherwise known as equalization can be done using different kind of algorithms [Non-patent Document 3].
As an example, in a typical Orthogonal Frequency Division Multiplexing (OFDM) based receiver [Non-patent Document 4], each subcarrier associated with each receive antenna of a given sector can be considered as a channel matrix H, resulting in a signal (vector) sk that is given bysk=Hk,ixk+nk,i  (1)where k is a subcarrier index that can take an integer value that is less than total number of available subcarriers, i is an index of a sector, Hk,i is a channel matrix at subcarrier k and sector with index i, xk is originally transmitted signal vector at subcarrier k and nk,i is a noise vector.
If we consider that Minimum Mean Square Error (MMSE) algorithm has been used to remove interference from received signal vector sk then the signal detected to have originated from the ith sector is given by (2)xk,i=(σ2I+Hk,iHHk,i)−1Hk,iHsk  (2)
In this expression, it has been assumed that channel matrix Hk,i of ith sector at the kth subcarrier has been estimated by a channel estimation unit. There are several techniques by which this channel matrix Hk,i can be estimated to an accurate level [Non-patent Document 8] [Patent Document 4]. Having detected the original transmitted signal from a given transmitting unit, the next step is to combine all the signals that have been detected at different sectors. One way of doing this, is to utilize an optimum combination algorithm such as MRC [Non-patent Document 5].
FIG. 2 illustrates a general case in a signal flow diagram where symbols detected separately are combined to create symbols for channel decoding, taking into consideration sector selection as described in [Patent Document 1]. The received signals at different sectors undergo preprocessing 11 and demodulation 12, respectively, and are combined by symbol combination 13 to generate a detected signal.
FIG. 3 is a block diagram showing the configuration of the system shown in FIG. 2. Referring to FIG. 3, two signal streams (25) and (26) are associated with sector-1. In addition, channel parameters (29) are used together with the two streams (Sector-1, signal x0, x1) (25) and (26) to perform interference cancellation in unit (21-1). Similarly, two signal streams (Sector-2, signal y0, y1) (27) and (28) that are associated with sector-2, are used together with channel parameters (30) to perform interference cancellation in unit (21-2). The resulting signals that have been generated from each sector and have interference cancelled are combined in combination unit (13). In practice, a detection unit (35) is also provided to generate information that is used to perform handover from one sector to another. Such kind of a handover is implemented using a selector (34) that select a signal among a combined signal (33), a signal from sector-1 (31) and a signal from sector-2 (32).
Detecting the location of a transmitting unit using the methods [Patent Document 1] mentioned earlier has its own disadvantage. Method based on signal to noise ratio involves generating a reference symbol and channel matrix, and this is computationally involving since it involves creating a kind of replica. Method based on BER involves independently decoding signals detected from each sector.
In all the above-mentioned related arts, the base station does not dynamically utilize efficiently all information that is received from signal associated with each sector. In addition, there is a necessity to implement a simple algorithm for detecting if signal from a target-transmitting unit is available within the received signal associated with a given sector.
[Non-Patent Document 1]
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